A conventional shape memory alloy is of crystal structure (referred to as “parent phase” hereinafter) at a relatively high temperature, which can be spontaneously transformed into another crystal structure (generally referred to as “martensitic phase”) at a relatively low temperature. A material is transformed from a parent phase into a martensitic phase when cooled from a high temperature to a low temperature, which is known as Martensitic Phase Transformation. Inversely, the material is transformed from a martensitic phase into a parent phase when heated from a low temperature to a high temperature, which is known as Inverse Martensitic Phase Transformation. Generally, start and end points of the martensitic phase transformation are respectively referred to as Ms and Mf points, and start and end points of the inverse martensitic phase transformation are respectively referred to as As and Af points. If the difference between Ms and As is small, for example, from several degrees Celsius to scores over 100° C., such martensitic phase transformation of the material is called Thermo-Elastic Martensitic Phase-Transformation.
In general, a certain kind of alloy material is cooled from a determined shape in a parent phase until a martensitic phase transformation occurs, then changing its original shape artificially, and subsequently, such alloy material is heated up until transformed back to the parent phase, and if the shape of the alloy material is completely or partially changed into its original shape, this phenomenon is called Shape Memory Effect. Moreover, in the same temperature cycle as above, if the shape in the parent phase is deformed at the moment of the phase transformation caused by the cooling and deformed again at the moment of the inverse phase transformation caused by the subsequent heating, and partially or completely transformed back to its original shape in the parent phase, this phenomenon is called Two-Way Shape Memory Effect.
Shape memory alloys are widely applied in various “smart” usages, such as various drivers, temperature sensitive elements, medical devices and high elastic brackets, and so on.
Conventional shape memory alloys, such as NiTi, have no magnetism. Magnetic shape memory alloys, such as Ni2MnGa, FePt, MnNiGe alloys and the like, possess new properties which the previous shape memory materials do not have, that is, the magnetic shape memory alloys not only have the shape memory property based on the martensitic phase transformation, but also the ferromagnetism. These materials are called magnetic shape memory alloys or magnetic phase-transformation materials, wherein the Heusler magnetic shape memory alloys, such as Ni2MnGa, are most typical and the main feature of this kind of materials is that the martensitic structural phase transformation and the magnetic structural phase transformation simultaneously occur (i.e., coupling of the structural phase transformation and the magnetic phase transformation). Some of this kind of materials could have the property that its martensitic phase transformation can be driven by an additional artificial magnetic field through optimization in performance. That is, they have not only the property of phase transformation of a conventional shape memory material, i.e., the martensitic phase transformation being driven by a temperature variation (heat energy) or an external stress (mechanical energy), but also the property of magnetic field-driven martensitic phase transformation. Due to the new property of magnetic field controllable phase transformation, this kind of magnetic shape memory alloys have more varied controllability and more popular applications, comparing to the previous conventional shape memory alloys. This kind of magnetic shape memory alloys can not only be used in actuators, temperature sensitive elements and high elastic materials, but also extended to magnetic sensing, electric sensing, magnetic driving, and magnetic refrigeration, and so on.
However, there are many disadvantages in the previous magnetic phase-transformation materials. The greatest disadvantage thereof is that the optimal materials having magnetic phase-transformation effect have poor mechanical property, such as toughness and deformation rate, due to comprising main group elements such as Ga, Sn, and Ge. For example, the compressive strength of the Heusler magnetic phase-transformation materials is about 350 MPa, and their deformation rate and toughness are nearly zero. These problems hinder the application of the current magnetic phase transformation materials in the above various aspects.
Therefore, there is a need for new magnetic phase-transformation materials with better mechanical properties.